Sustainable Soybean Production Using Residual Vermicompost Inputs in Corn-Soybean Rotation

Abstract

Soybeans (Glycine max L.), a globally significant crop, play a critical role in economic, nutritional, and ecological systems, particularly in rotational farming due to their nitrogen-fixing capacity. This study investigated the residual effects of vermicompost (VC) and vermicompost tea (VCT) applied during a preceding corn cycle on subsequent soybean growth and productivity in an organic corn–soybean rotation. Soybeans were grown in raised beds previously treated with different VCT concentrations and combinations of VC+VCT, without additional fertilization during the soybean phase. Physiological traits, including leaf chlorophyll content (SPAD values) and stomatal conductance, were measured alongside biomass, yield, and plant leaves nutrient concentrations. VC+VCT treatments significantly increased biomass and yield, with VC1+VCT20 achieving the highest biomass (3.02 tons/ha) and yield (1.68 tons/ha). Leaf nutrient analysis revealed increased uptake of both macro- and micronutrients in amended treatments, while SPAD and stomatal conductance values remained consistently higher than in the control. Soil analyses confirmed improved nutrient retention and cation exchange capacity in amended plots, demonstrating the legacy benefits of organic inputs. Therefore, residual VCT and VC+VCT applications improved soybean productivity, nutrient acquisition, and physiological performance in rotational systems. By reducing reliance on synthetic fertilizers and enhancing soil fertility, this strategy supports climate-smart agriculture principles and contributes to SDG 2 (Zero Hunger), SDG 12 (Responsible Consumption and Production), and SDG 13 (Climate Action).

1. Introduction

Soybeans is a cornerstone of global agriculture, recognized as one of the most economically and nutritionally important crops worldwide. Its seeds are a rich source of amino acids, high protein content, oil, and essential amino acids [1,2] making it indispensable for human diets and livestock feed formulations. Beyond its role in food and feed, soybean-derived products contribute to diverse industrial applications, including biofuels, bioplastics, and pharmaceuticals, thereby reinforcing its strategic importance in sustainable development and the bioeconomy [2,3]. Global production is highly concentrated, with Argentina, Brazil, and the United States collectively accounting for over 80% of the total output [4]. In the United States alone, soybeans generated revenues exceeding USD 56 billion in 2023, and this demonstrates their critical role in rural economies, food security, and biofuel supply chains [4,5].

Beyond their economic value, soybeans are integral to sustainable agriculture due to their unique biological and ecological functions. Soybeans form symbiotic associations with nitrogen-fixing bacteria, thereby enriching the soil’s nitrogen content and reducing dependence on synthetic fertilizers [6,7]. This biological nitrogen fixation not only supports soybean growth but also benefits subsequent crops in rotations, enhancing soil fertility and productivity. As a result, soybeans serve as a cornerstone in crop rotations, where they can improve soil health and productivity for subsequent crops [8,9]. By reducing reliance on synthetic fertilizers, soybean-based systems contribute to climate-smart agriculture goals of lowering greenhouse gas emissions and building soil carbon stocks [10,11], which directly support SDG 2 (Zero Hunger) and SDG 13 (Climate Action).

Despite these advantages, optimizing yields of soybean in low-input or organic systems remains a challenge, primarily due to the need for sustained nutrient availability throughout the growing season without synthetic agrochemical inputs [12]. VC and VCT are promising organic amendments for sustainable nutrient management [13]. VC is a nutrient-dense, slow-release organic material that enhances soil structure, microbial activity, and long-term fertility [14,15]. VCT, a liquid extract of VC, delivers readily available nutrients and bioactive compounds that can improve the uptake efficiency of nutrients and enhance plant growth [16,17].

While the direct benefits of VC and VCT on crop performance are well documented, their residual effects, particularly in rotational systems where no further amendments are applied, remain poorly understood [18]. Soybeans, with their nitrogen-fixing ability and tolerance to moderate nutrient stress, are ideal for assessing these legacy effects. Understanding how residual VC and VCT influence soybean physiology, nutrient uptake, and yield can guide more efficient organic nutrient management. Such strategies align with climate-smart agriculture principles by improving nutrient use efficiency, maintaining productivity under input constraints, and enhancing agroecosystem resilience [19,20], which are key targets under SDG 12 (Responsible Consumption and Production) and SDG 15 (Life on Land) [21]. This is particularly relevant for organic systems, where nutrient availability is largely governed by the mineralization dynamics of previously applied organic matter.

The novelty of this study lies in its focus on the residual effects of VC and VCT in rotational systems, specifically evaluating their impact on soybean performance following a preceding corn cycle without additional fertilization. Therefore, this study evaluates the residual effects of VC and VCT applied in a preceding corn cycle on soybean growth, physiology, and productivity. Specifically, we examined their effects on chlorophyll content, stomatal conductance, biomass, and yield. We hypothesize that (1) residual VC and VCT would enhance soybean performance compared to untreated controls, and (2) combined VC+VCT treatments would have the most potent effects because of their complementary nutrient release patterns. The findings aim to advance our understanding of organic nutrient cycling in crop rotations and support the development of resilient, low-input production systems that are both environmentally sustainable and agronomically viable.

2. Materials and Methods

2.1. Site Description and Experimental Design

The study was conducted at the Agroecology Program’s organic garden (coordinates: 25.754906, −80.380032) at Florida International University, Miami, Florida, which is managed exclusively under organic practices and has calcareous sandy loam soils typical of South Florida agroecosystems. This trial followed our preceding corn experiment [22], utilizing the same raised beds without any additional fertilizer or amendments to assess the residual effects of previous applications.

Soybean seeds (black variety) were obtained from Hudson Valley Seed Company, located in Accord, NY, United States, and were sown in the first week of September 2024, two days post-corn harvest, and harvested in December 2024. Weather data from the official NOAA climate portal (https://www.weather.gov/wrh/Climate?wfo=mfl accessed on 23 July 2025) was used to obtain data on weather for the study area during this period. Monthly mean precipitation was 7.62 mm (September), 6.10 mm (October), 0.51 mm (November), and 1.02 mm (December). Average temperatures ranged from 29.2 °C (September) to 22.1 °C (December).

Seven galvanized steel planting beds (L × W × H: 243.84 × 60.96 × 30.48 cm), sourced from a commercial supplier (Vego Garden) in Houston, TX, USA, were installed over a layer of wood mulch and weed-suppressing fabric (Agfabric, Gardenport, Corona, CA, USA) in an open-air field exposed to natural environmental conditions. These beds, previously used for a corn cultivation trial, were filled with a potting substrate provided by RGS Nursery (Miami, FL, USA) and retained the same soil composition for the subsequent soybean experiment.

The treatments applied during the corn phase were carried over into the soybean cycle and included the following: V0: untreated control with no VC or VCT; VCT10: 10% VCT; VCT20: 20% VCT; VCT40: 40% VCT; VC1+VCT10: VC at 1 ton/acre combined with 10% VCT; VC1+VCT20: VC (1 ton/acre) and 20% VCT; VC1+VCT40: VC (1 ton/acre) and 40% VCT. In the corn phase, VCT was applied weekly for six weeks (100 mL per plant) via foliar and drench spray starting at the V2 corn stage, while VC was applied once before planting. These same treatments were carried over into the soybean phase without reapplication, allowing assessment of residual effects. The treatment rates were based on prior optimization studies [23] and represent agronomically and economically realistic levels for organic systems. Each raised bed was divided into three replicates, and irrigation was provided daily, except during rainfall events.

The VC used in this study was produced from mushroom waste blocks sourced from Lions Fruit Farm (Miami, FL, USA). The composting process involved Eisenia fetida earthworms over a four-month period, during which organic matter was decomposed through mechanical fragmentation, enzymatic activity, and microbial processes [13]. VCT was prepared by aerating steeped VC in water for 24 h, then diluting to the desired concentrations (10%, 20%, and 40%) for weekly foliar and drench applications.

2.2. Plant Growth and Physiology Measurements

Leaf chlorophyll content was measured weekly between 9:00 am and 10:30 am from 32 to 60 days after sowing (DAS) using a SPAD 502 Plus chlorophyll meter from Spectrum Technologies Inc., located in Aurora, IL, USA, as per the established protocol by [23]. Stomatal conductance was measured during the same period using a LI-600 fluorometer/porometer manufactured by LI-COR Biosciences, based in Lincoln, NE, USA. These physiological parameters were selected to assess plant health and photosynthetic efficiency under residual nutrient conditions.

2.3. Plant and Soil Sampling Procedures

At 77 days after sowing (DAS), entire soybean plants, including pods and roots, were collected and airdried at a temperature range of 25–30 °C until a constant weight was achieved, and the total biomass was recorded. Grain yield was determined by weighing the dried grains (excluding pods) and calculating yield per hectare based on the raised bed using the following formula:

$$\mathrm{S}\mathrm{o}\mathrm{y}\mathrm{b}\mathrm{e}\mathrm{a}\mathrm{n} \mathrm{g}\mathrm{r}\mathrm{a}\mathrm{i}\mathrm{n} \mathrm{y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d} \mathrm{i}\mathrm{n} \mathrm{t}\mathrm{o}\mathrm{n}\mathrm{s}/\mathrm{h}\mathrm{a}={\displaystyle \frac{\mathrm{T}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l} \mathrm{d}\mathrm{r}\mathrm{y} \mathrm{g}\mathrm{r}\mathrm{a}\mathrm{i}\mathrm{n} \mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t} \mathrm{i}\mathrm{n} \mathrm{t}\mathrm{o}\mathrm{n}\mathrm{s}}{\mathrm{A}\mathrm{r}\mathrm{e}\mathrm{a} \mathrm{o}\mathrm{f} \mathrm{t}\mathrm{h}\mathrm{e} \mathrm{r}\mathrm{a}\mathrm{i}\mathrm{s}\mathrm{e}\mathrm{d} \mathrm{b}\mathrm{e}\mathrm{d} \mathrm{i}\mathrm{n} {\mathrm{m}}^2}} \times \mathrm{10,000}$$

Dried leaves from each replicate were grinded into powder using Hamilton Beach coffee grinder (Glen Allen, VA, USA), and were analyzed for macronutrients (N, K, C, Mg, P, S, Ca) and micronutrients (B, Mn, Na, Zn, Fe, Cu) using a C/N analyzer and using inductively coupled plasma mass spectrometry (ICP-MS), in accordance with the methodology outlined by [22]. Detection limits for ICP-MS were N (0.01%), P (0.005%), K (0.005%), Ca (1 mg/kg), Mg (1 mg/kg), Zn (0.1 mg/kg), Cu (0.1 mg/kg), Fe (0.1 mg/kg), Mn (0.1 mg/kg), B (0.1 mg/kg), and Na (0.01%).

Soil samples were collected on the same day from two depths in each replicate: surface (0–10 cm) and subsurface (10–20 cm). Samples were air-dried and sieved (2 mm) prior to analysis. Soil pH was assessed using a 0.01 M calcium chloride solution. The concentrations of carbon and nitrogen were measured with a C/N analyzer. Macro- and micronutrient levels were determined via inductively coupled plasma mass spectrometry (ICP-MS). Organic matter (OM) content was estimated using the loss-on-ignition technique, and cation exchange capacity (CEC) was calculated based on the total exchangeable charges of H, Zn, Na, Ca, and K, following the method outlined by [22]. Detection limits for ICP-MS in soil samples were approximately P (0.005%), K (0.005%), Ca and Mg (1 mg/kg), Na and S (0.01%), Fe (0.3 mg/kg), Zn (0.4 mg/kg), Cu (0.6 mg/kg), Mn (0.3 mg/kg), and B (2.2 mg/kg).

2.4. Statistical Analysis

Normality of the data was assessed using the Shapiro–Wilk test, and variance homogeneity was evaluated with Levene’s test. In cases where these assumptions were not met, suitable data transformations or non-parametric approaches were employed. All statistical analyses were performed using Minitab version 22 (Minitab LLC, State College, PA, USA). A one-way ANOVA was conducted at a 5% significance level. When the overall F-test indicated significance and ANOVA assumptions were satisfied, Tukey’s HSD post hoc test was applied to compare treatment means, ensuring control of the family-wise error rate during multiple pairwise comparisons.

3. Results

3.1. Soil Physicochemical Properties

To evaluate the carryover effects of the corn-phase treatments, baseline soil nutrient data were collected immediately after the corn harvest [22] (

Table 1. Physicochemical properties of soils following corn harvest and subsequent application of vermicompost (VC) and vermicompost tea (VCT). The data adapted from [22] reflect residual soil fertility status prior to soybean cultivation.

Treatment	pH	N (%)	P (%)	K (%)	Ca (mg kg−1)	Mg (mg kg−1)	Zn (mg kg−1)	Na (mg kg−1)	C (%)	OM (%)	CEC (cmol kg−1)
VC a	7.5	1.55	0.176	0.502	1831	1391	30.5	296	22.8	39.3	33.6
Surface soils (0–10 cm)
V0	7.83 a	0.09 b	0.002 b	0.001 b	267 b	43 b	1.6 b	2.0 b	1.80 c	3.1 c	1.7 b
VCT10	7.77 a	0.69 a	0.018 a	0.010 a	2517 a	385 a	17.0 a	19.5 a	12.4 ab	21.3 ab	16.0 a
VCT20	7.83 a	0.80 a	0.017 a	0.008 a	2578 a	368 a	17.0 a	18.7 a	14.2 ab	24.4 ab	16.2 a
VCT40	7.77 a	0.86 a	0.016 a	0.009 a	2694 a	373 a	14.1 a	17.2 a	17.6 a	30.2 a	16.8 a
VC1+VCT10	7.77 a	0.72 a	0.018 a	0.009 a	2676 a	372 a	16.3 a	15.2 a	13.1 ab	22.6 ab	16.7 a
VC1+VCT20	7.9 a	0.62 a	0.017 a	0.008 a	2579 a	356 a	15.1 a	21.2 a	11.0 b	18.9 b	16.1 a
VC1+VCT40	7.83 a	0.85 a	0.019 a	0.010 a	2641 a	397 a	15.7 a	21.8 a	15.0 ab	25.8 ab	16.8 a
Subsurface soils (10–20 cm)
V0	7.87 ab	0.08 b	0.002 c	0.002 c	237 b	55 b	1.5 b	3.3 b	1.5 c	2.6 c	1.7 c
VCT10	7.87 ab	0.88 a	0.018 ab	0.009 b	2409 a	432 a	13.8 a	17.6 a	15.4 ab	26.4 ab	15.9 ab
VCT20	7.80 ab	0.77 a	0.018 ab	0.009 b	2505 a	457 a	14.6 a	19.3 a	13.5 ab	23.1 ab	16.6 ab
VCT40	7.73 b	0.83 a	0.017 b	0.010 b	2416 a	503 a	15.4 a	21.7 a	16.1 a	27.8 a	16.5 a
VC1+VCT10	7.93 ab	0.69 a	0.019 ab	0.011 b	2512 a	488 a	15.1 b	20.9 a	12.0 b	20.6 b	16.9 b
VC1+VCT20	7.93 ab	0.78 a	0.024 a	0.018 a	2609 a	590 a	16.6 a	25.0 a	13.7 ab	23.6 ab	18.4 ab
VC1+VCT40	7.97 a	0.70 a	0.019 ab	0.012 b	2442 a	474 a	20.0 a	21.8 a	12.2 b	21.0 b	16.5 b

Values represent mean (n = 3 per treatment). Treatments assigned different letters are significantly different from each other at the 0.05 probability level. ^a The VC data provided reflect applications made during the corn experiment.

). Soils at both sampling depths exhibited slightly alkaline conditions, with pH values ranging between 7.77 and 7.97. Nutrient levels of N, P, and K were generally consistent across treatments, except for the control group (V0), which consistently showed significantly reduced concentrations. Comparable patterns were noted for OM, CEC, and the majority of macro- and micronutrients.

For post-soybean harvest soil data (

Table 2. Nutrient content of soils from two depths post soybean harvest.

Treatment	pH	N (%)	P (%)	K (%)	Ca (mg kg−1)	Mg (mg kg−1)	Zn (mg kg−1)	Na (mg kg−1)	C (%)	OM (%)	CEC (cmol kg−1)
Surface soil (0–10 cm)
V0	7.57 a	0.41 c	0.011 c	0.006 b	1761 c	283 c	10.84 c	11.16 b	8.13 b	13.98 b	11.32 c
VCT10	7.40 a	0.65 b	0.020 ab	0.010 ab	2816 b	377 ab	19.37 a	19.52 ab	12.41 ab	21.34 ab	17.48 b
VCT20	7.50 a	0.69 b	0.019 ab	0.012 ab	2856 ab	369 b	18.45 ab	24.62 a	12.74 ab	21.91 ab	17.66 b
VCT40	7.43 a	0.73 b	0.018 b	0.012 ab	2857 ab	450 a	16.53 b	26.00 a	14.92 b	25.66 b	18.33 ab
VC1+VCT10	7.07 a	0.70 b	0.024 a	0.014 a	3095 a	425 ab	19.84 a	28.34 a	12.41 ab	21.34 ab	19.38 a
VC1+VCT20	7.33 a	0.65 b	0.022 ab	0.012 a	2990 ab	384 ab	19.59 a	25.30 a	11.70 ab	20.13 ab	18.46 ab
VC1+VCT40	7.37 a	0.62 ab	0.022 ab	0.015 a	2974 ab	394 ab	19.21 a	25.53 a	11.04 ab	18.98 ab	18.53 ab
Subsurface soils (10–20 cm)
V0	7.50 a	0.43 b	0.012 b	0.005 b	1749 b	320 b	10.63 b	8.09 b	8.22 b	14.14 b	11.54 b
VCT10	7.40 a	0.70 a	0.024 a	0.008 a	2910 a	481 a	19.14 a	11.91 ab	12.57 a	21.63 a	18.78 a
VCT20	7.47 a	0.73 a	0.020 a	0.009 a	3086 a	498 a	17.98 a	13.60 a	13.73 a	23.61 a	19.80 a
VCT40	7.57 a	0.71 a	0.019 a	0.008 a	2991 a	534 a	17.08 a	13.81 a	13.38 a	23.02 a	19.60 a
VC1+VCT10	7.47 a	0.66 a	0.023 a	0.010 a	3032 a	504 a	18.39 a	12.97 a	12.16 a	20.91 a	19.63 a
VC1+VCT20	7.37 a	0.72 a	0.024 a	0.009 a	2919 a	499 a	18.71 a	10.79 ab	13.28 a	22.84 a	18.97 a
VC1+VCT40	7.40 a	0.69 a	0.023 a	0.010 a	2973 a	484 a	19.03 a	11.87 ab	12.33 a	21.21 a	19.15 a

Note: Means marked with different letters indicate statistically significant differences at p < 0.05.

), nutrient patterns in surface soils (0–10 cm) largely mirrored baseline trends: all VCT and VC+VCT treatments had significantly higher N, P, C, and OM compared to V0. The control consistently showed the lowest nutrient concentrations and CEC values. Statistical comparisons revealed no statistically significant differences between VC-only and VC+VCT treatments for most nutrients. However, VCT40 had significantly lower P than VC1+VCT40, higher Mg than VCT20, and lower Zn than most other treatments (except VCT20). For CEC, VCT10 and VCT20 were significantly lower than VC1+VCT10.

In subsurface soils (10–20 cm), the control treatment V0 again had the lowest nutrient concentrations, OM, and CEC. Differences between VCT and VC+VCT treatments were generally non-significant for most elements. While nutrient levels were broadly similar between surface and subsurface soils, K and Na exhibited notable vertical variation: surface soils contained 20–50% more K and 40–130% more Na than subsurface soils.

Comparing baseline data (Table 1) and post-soybean harvest data (Table 2) revealed substantial increases in nutrients, especially in the control bed. In surface soils, pH shifted from slightly alkaline to near neutral (7.07–7.57). In V0 treatment, N, C, and OM increased nearly fivefold, while Na increased sixfold, and P, K, Ca, Mg, Zn, and CEC increased about sevenfold relative to baseline. Specific treatments also showed notable increases: K doubled in VCT20, VC1+VCT20, VC1+VCT40, and VC1+VCT10, while Na doubled in VCT40 and VC1+VCT10 compared to their baseline levels. Other nutrients showed minimal changes between baseline and post-harvest values.

Similar patterns were observed in subsurface soils. Compared to baseline, post-harvest V0 soils had about five times more N, C, and OM; about six times more P and Mg; about seven times more Ca, Zn, and CEC; about three times more K; and about two times more Na. An exception was VC1+VCT40, where Na levels dropped to less than half of baseline. For other nutrients, changes between baseline and post-harvest values were negligible.

3.2. Plant Nutrient Content

The soybean leaves’ nutrient content shows enhanced macro- and micronutrient uptake in soils previously amended with VC and VCT compared to the control (V0) (

Table 3. Nutrient composition of soybean leaves under VCT and VC+VCT treatments.

Sample	N (%)	P (%)	K (%)	Ca (%)	Mg (%)	S (%)	Na (%)	B (mg kg−1)	Fe (mg kg−1)	Mn (mg kg−1)	Zn (mg kg−1)	Cu (mg kg−1)
V0	1.00 f	0.74 d	0.40 e	1.90 e	0.36 e	0.07 b	0.04 a	42.85 g	43.06 g	22.55 f	165.85 f	7.22 d
VCT10	1.56 d	1.71 b	0.82 b	2.97 d	0.63 bc	0.14 a	0.05 a	67.46 e	101.78 b	37.56 b	259.43 e	7.32 d
VCT20	1.95 a	1.58 c	0.58 c	2.99 d	0.56 d	0.13 ab	0.05 a	65.23 f	86.25 d	35.64 d	284.42 b	9.71 b
VCT40	1.87 b	1.79 a	0.80 b	3.01 cd	0.57 d	0.11 ab	0.05 a	70.80 c	81.30 e	32.55 e	315.64 a	7.39 cd
VC1+VCT10	1.62 c	1.56 c	0.60 c	3.29 b	0.67 b	0.15 a	0.05 a	78.50 b	77.25 f	36.77 c	262.41 d	7.90 c
VC1+VCT20	1.39 e	1.70 b	0.51 d	3.74 a	0.76 a	0.14 a	0.04 a	82.22 a	88.25 c	43.61 a	259.51 e	6.99 d
VC1+VCT40	1.53 d	1.58 c	0.88 a	3.05 c	0.59 cd	0.12 ab	0.06 a	68.19 d	109.80 a	37.87 b	271.44 c	10.71 a

Mean values followed by different letters differ significantly at p < 0.05.

). All measured nutrients, except Na, varied significantly across treatments. Na concentrations remained statistically unchanged among treatments, including the control.

For every nutrient except Na, V0 consistently had the lowest leaf concentrations. The highest N (1.95%), P (1.79%), and K (0.88%) contents were recorded in VCT20, VCT40, and VC1+VCT40 treatments, respectively. VC1+VCT20 had the highest Mg, B, Ca, and Mn contents, while Fe and Cu were highest in VC1+VCT40, and Zn was highest in VCT40. There was no statistically significant difference in sulfur levels between the VCT and VC+VCT treatments. However, V0 had significantly lower S levels than VCT10, VC1+VCT10, and VC1+VCT20.

When the ratio of soybean leaf nutrient concentration to residual soil nutrient concentration was calculated (Figure 1), distinct patterns in nutrient uptake efficiency were observed. In V0 treatment, K and P uptake efficiencies were relatively high despite low absolute nutrient content. Increasing VCT concentration enhanced P and K uptake efficiency, with VCT40 showing the highest P ratio (96.8) and a high K ratio (80.0). N uptake ratios remained relatively stable across VCT-only treatments (2.3–2.7) while VC+VCT treatments generally reduced K uptake efficiency, with the lowest values in VC1+VCT20 (48.6) and VC1+VCT10 (50.0). P uptake ratios followed a similar trend, remaining high in VC1+VCT20 but not surpassing those in VCT-only treatments. Mg uptake efficiency peaked in VC1+VCT20, while Na uptake efficiency was highest in V0 and VC1+VCT40 (Figure 1).

3.3. Relative Chlorophyll Content

SPAD measurements showed a general increase in leaf chlorophyll content with advancing plant maturity across all treatments (Figure 2). Across all sampling dates, the control treatment (V0) consistently recorded significantly lower SPAD values compared to both VCT-only and VC+VCT treatments. SPAD readings in V0 ranged from 11 to 15 throughout the experiment, showing minimal fluctuation. No significant differences were observed between VCT-only and VC+VCT treatments, except in the case of VCT40, which showed marginally higher readings from 32 to 60 DAS.

3.4. Stomatal Conductance

Stomatal conductance increased with plant maturity across all treatments (Figure 3). VC+VCT treatments consistently exhibited higher conductance than VCT-only and control (V0) treatments, indicating enhanced water regulation and physiological activity. Among these, VC1+VCT20 and VC1+VCT40 frequently recorded the highest conductance throughout the experiment. At 24 DAS, VC+VCT treatments already showed significantly greater conductance than both VCT-only and V0 treatments. VCT-only treatments increased gradually between 24 and 32 DAS, then remained relatively stable until 60 DAS, with the exception of VCT10, which showed a sharp increase between 53 and 60 DAS. The control (V0) maintained the lowest stomatal conductance throughout the experimental period, suggesting limited physiological responsiveness under nutrient-deficient conditions.

3.5. Biomass and Yield

Nutrient carryover from the organic amendments had a significant influence on soybean biomass and yield (Figure 4). In terms of total biomass, VC+VCT treatments generally outperformed both VCT-only and V0, demonstrating the synergistic benefits of combined organic inputs. An exception was VCT20, which produced more biomass (2.36 tons/ha) than VC1+VCT10 (2.20 tons/ha). The highest biomass was recorded in VC1+VCT20 (3.02 tons/ha), whereas VCT40 yielded significantly less biomass than all other VCT-only and combined VC and VCT treatments. V0 had the least biomass (1.02 tons/ha), reflecting the absence of residual nutrient support.

Yield trends closely mirrored biomass patterns. VC1+VCT20 and VC1+VCT10 achieved the highest yields at 1.68 tons/ha and 1.37 tons/ha, respectively. VCT20 and VCT10 also performed well, producing higher yields than VC1+VCT10 and VCT40. VCT40 had the lowest yield among the VCT and VC+VCT treatments (0.97 tons/ha), while V0 remained the lowest overall.

4. Discussion

4.1. Soil Physicochemical Properties

Baseline nutrient content (Table 1) showed a slightly alkaline pH (7.77–7.97), consistent with the calcareous sandy loam soils of South Florida [23]. The lack of significant differences in N, P, K, and most other nutrients across treatments, except in the control (V0), indicates that nutrient contributions from VC and VCT amendments applied during the corn phase persisted into the soybean cycle. This persistence suggests that organic amendments can maintain soil fertility across crop cycles, even without reapplication. Because the experiment was conducted in raised beds under controlled daily irrigation in Miami’s hot and humid climate, rainfall variability was unlikely to influence soil moisture or nutrient leaching. Therefore, the observed differences are attributable to residual amendments rather than environmental factors. The findings of this study align with findings by [24], who reported that organic amendments and crop residue returns enhance nutrient retention and aggregate stability in rotational systems.

Post-harvest analysis (Table 2) showed that surface soils (0–10 cm) under organic input treatments retained significantly higher levels of N, P, C, organic matter, and CEC compared to V0, which remained lowest in all parameters. These results confirm that residual fertility from the corn phase carried over into soybean production, reinforcing the long-term benefits of organic inputs. The consistently warm conditions in Miami may also have supported microbial activity and organic matter mineralization, enhancing nutrient release from residual VC and VCT [25]. Higher nutrient levels in amended treatments may also reflect synergistic effects from soybean root exudates and rhizosphere microbial processes, which are known to stimulate nutrient cycling and retention. This is consistent with the way VC improves nutrient retention and CEC through its high OM and humic content [16,26].

In some cases, nutrient levels were similar between VCT-only and VC+VCT treatments, suggesting that VCT alone can sustain nutrient availability. However, VCT40 showed significantly lower P and Zn compared to VC1+VCT40, indicating that higher VCT concentrations alone may not improve nutrient retention and could even lead to nutrient imbalances. Under the controlled moisture conditions of this study, such differences are unlikely to result from rainfall-driven leaching. Instead, they may reflect treatment-specific nutrient dynamics such as microbial immobilization, plant uptake, or chemical interactions [27]. For example, lower P in VCT40 may result from precipitation with Ca in high-pH soils or increased plant uptake. Reduced Zn in VCT40 could be due to greater removal in biomass, given the mobility of Zn in legumes. Conversely, higher Mg in VCT40 compared to VCT20 may be attributed to solubilization from organic matter, highlighting the complex interactions between amendment concentration and nutrient availability.

Nutrient stratification was evident, with surface soils containing 20–50% more K and 40–130% more Na than subsurface layers. This likely reflects the carryover effect from foliar and drench VCT applications during corn growth, which concentrated nutrients near the soil surface. Soybean root systems, known for active nutrient cycling in the upper soil profile under organic management [28,29], likely reinforced this stratification. Our findings are consistent with [30], who reported that legume integration in crop rotations significantly boosts soil organic carbon and nutrient cycling, particularly in organic systems where synthetic inputs are absent.

When comparing baseline to post-soybean soils, even the control (V0) showed nearly fivefold increases in N, C, and OM in surface soils, despite no added inputs. Although V0 showed significant relative increases compared to its own baseline, these values remained substantially lower than all amended treatments. The observed increases in V0 reflect biological nitrogen fixation by soybeans and enhanced microbial activity stimulated by root exudates during the growing period [31,32]. These findings support the concept that legume crops can improve soil fertility through symbiotic nitrogen fixation and rhizosphere-driven nutrient cycling, even in nutrient-poor soils [31,32,33]. This is also consistent with the “soybean nitrogen credit” concept, where legumes enhance N availability for subsequent crops [27,34].

Notably, treatments such as VCT20 and VC1+VCT40 doubled K and Na post-harvest, demonstrating synergistic effects between residual corn-phase amendments and soybean-driven nutrient cycling. The slight pH drop from alkaline to near-neutral likely resulted from organic acid release via soybean root exudates and microbial activity, a well-documented phenomenon in organically managed legume systems [35,36,37].

4.2. Plant Chemical Analysis

Soybean leaf nutrient content and uptake efficiency patterns indicated apparent legacy effects of VCT and VC applications from the corn phase (Table 3). Since no inputs were applied during soybean growth, the observed differences in nutrient profiles are attributable to residual soil fertility and chemical transformations from prior treatments.

Post-harvest soil data confirmed that VCT and VC+VCT treatments maintained significantly higher nutrient reserves than V0, and these reserves translated into higher leaf macro- and micronutrient content. This supports previous findings that VC-based amendments enhance nutrient retention and mineralization over time, thereby increasing nutrient availability for subsequent crops [38,39].

The sodium content showed no significant treatment effect, likely reflecting its minor role in soybean nutrition and limited variability in soil Na. However, higher Na uptake ratios in V0 and VC1+VCT40 may indicate osmotic adjustment under nutrient-limited or ion-rich conditions, where Na accumulation helps maintain cell turgor or reflects elevated soluble salt levels [40,41,42]. Furthermore, nutrient-specific patterns also revealed interesting findings. The high N content in VCT20 suggests optimal mineralization without excessive loss. In contrast, the peak K content in VC1+VCT40 likely reflects contributions from both the mineral fraction of VC and the soluble K in VCT. The elevated Ca, Mg, B, and Mn content in VC1+VCT20 (Table 3) points to synergistic improvements in CEC and divalent cation availability. Furthermore, Fe, Zn, and Cu were highest in VC1+VCT40 and VCT40, consistent with the ability of VC to chelate micronutrients and enhance plant uptake [39,43].

Uptake efficiency ratios, such as high K and P ratios in V0 (Figure 1), despite low absolute nutrient levels, suggest compensatory uptake mechanisms, where plants in nutrient-limited soils enhance root allocation and scavenging capacity [44,45]. However, yield in V0 remained constrained by the small total nutrient pool. Increasing VCT rates boosted P and K uptake efficiencies, with VCT40 reaching the highest P ratio (96.8) and maintaining strong K uptake (80.0). This suggests that VCT promotes more bioavailable nutrient forms, likely through the release of organic acids and microbial solubilization processes [46]. By contrast, VC+VCT treatments often had lower K uptake ratios, possibly due to a dilution effect from higher total K availability or cation competition in soils enriched with Ca²⁺ and Mg²⁺ [43,47]. The highest Mg uptake ratios in VC1+VCT20 aligned with leaf Mg results, reinforcing the advantage of combining solid and liquid organic amendments for sustained nutrient supply.

4.3. Relative Chlorophyll Content

SPAD readings increased across all treatments from 32 to 60 DAS (Figure 2), indicating normal canopy development and chlorophyll accumulation during soybean vegetative growth. Since SPAD values serve as a proxy for leaf chlorophyll and nitrogen status [48], the consistently lower readings in the control (V0) indicate limited nitrogen availability, which is consistent with soil and leaf analyses showing significantly lower N content in V0 compared to the VCT and VC+VCT treatments. This highlights the importance of residual organic inputs in sustaining early-season nitrogen availability and chlorophyll synthesis.

Among the amended treatments, VCT40 and VC1+VCT20 maintained higher SPAD values throughout the growth period, indicating a nutrient environment that supported prolonged chlorophyll production. VCT likely contributed soluble nutrients and microbial stimulants from the corn phase, while VC provided slow-release nutrients that continued to nourish the soybean crop [13,23]. The absence of substantial differences among amended treatments (Figure 2) suggests a nutrient threshold effect, where once adequate N is available, additional inputs do not further enhance chlorophyll levels. This plateau may reflect nutrient saturation or uniform root uptake efficiency as plants mature.

By 60 DAS, SPAD readings converged across treatments, likely due to nutrient uptake stabilizing and internal redistribution as the crop transitioned to reproductive growth. At this stage, chlorophyll levels typically plateau or decline as nutrients are remobilized to developing pods. Decomposition of organic residues and microbial activity may also have evened out nutrient availability across treatments. These patterns are consistent with previous studies showing that organic amendments boost early chlorophyll development but have diminishing effects as plants enter reproductive stages [43,49].

4.4. Stomatal Conductance

Stomatal conductance increased over time across all treatments (Figure 3), reflecting progressive canopy development and rising photosynthetic demand. The highest conductance values were observed in VC+VCT treatments, particularly VC1+VCT20 and VC1+VCT40, suggesting that the combination of slow-release nutrients from VC and bioactive compounds in VCT created an optimal soil environment for root development and water uptake. This likely enhanced the plants’ ability to regulate stomatal opening, a process closely linked to water availability, nutrient status, and overall physiological vigor [46,50].

Bioactive components in VCT, such as humic substances, cytokinins, and auxins, may have stimulated stomatal opening by enhancing photosynthetic activity and internal water transport [13,23]. However, without a steady nutrient supply from VC, the effects of VCT alone appeared less durable. VCT-only treatments showed moderate early increases in stomatal conductance but plateaued from 32 to 60 DAS, except for VCT10, which exhibited a late-season increase. This pattern suggests that VCT can provide a short-term physiological boost, but VC is necessary to maintain the effect throughout the growth cycle [22]. The consistently low stomatal conductance in V0 shows how nutrient limitations can restrict root development, water uptake, and stomatal activity, ultimately affecting photosynthetic performance.

4.5. Biomass and Yield

Soybean biomass and yield were strongly influenced by residual amendments from the corn phase (Figure 4). V0, which received no prior inputs and recorded the lowest biomass and yield due to nutrient-deficient conditions. In contrast, VC+VCT treatments consistently outperformed both VCT-only and control treatments, with VC1+VCT20 producing the highest biomass and yield. These results highlight the synergistic effects of combining sustained nutrient release of VC with VCT’s fast-acting nutrients and bioactive compounds, which together enhance early growth, microbial activity, and nutrient availability [22,26]. Soybean yields in our VC1+VCT20 treatment (1.68 tons/ha) were comparable to those reported by [51] in short-term organic rotation using biological fertilizers, suggesting that residual organic inputs can match the performance of actively fertilized organic systems.

Notably, VCT20 outperformed VC1+VCT10 in biomass, indicating that optimal VCT rates can rival combined treatments under certain conditions. However, VCT40 yielded significantly less biomass and grain yield than other amended treatments, suggesting that high concentrations may lead to nutrient imbalances, reduced uptake efficiency, or mild phytotoxic effects [22]. These findings align with prior research, which shows that the benefits of VCT are concentration-dependent and may plateau or decline at an excessive rate [16].

While VCT10 and VCT20 delivered respectable yields without VC, the absence of a sustained nutrient source likely limited total biomass accumulation. As expected for a no-fertilizer soybean phase, overall yields were lower than the commercial average obtained by farmers in the state of Georgia, USA (10.63 tons/ha) [52]. Nevertheless, the results demonstrate that residual organic amendments from a previous crop can provide meaningful yield gains in low-input systems. Our findings support the broader environmental benefits of organic soybean systems, consistent with [53], who reported that organic rotations reduced environmental impact by over 30% compared to conventional systems.

5. Conclusions

This study demonstrated that nutrient effects from prior applications of compost-based inputs during the preceding corn cycle can substantially improve soybean performance in an organic rotation system. By tracking plant physiology, nutrient uptake, and yield, we found that combining VC and VCT, especially at moderate rates, provided both immediate and sustained nutrient supply. This translated into higher chlorophyll content, stronger stomatal conductance, greater biomass, and improved yields.

VC1+VCT20 treatment emerged as the most effective treatment, demonstrating that a balanced combination of solid and liquid organic amendments can optimize nutrient use efficiency and promote vigorous plant growth. In contrast, higher VCT rates, such as VCT40, were less effective, highlighting the importance of avoiding nutrient imbalances. Even without amendments, soybeans maintained a baseline yield, demonstrating their resilience and nitrogen-fixing ability.

These results demonstrate the value of incorporating residual organic amendments into crop rotation systems to reduce dependence on chemical inputs, improve soil quality, and promote environmentally friendly farming practices. Future work should evaluate these approaches across multiple rotation cycles, diverse agroecological settings, and long-term multi-season trials to assess their consistency, resilience, and scalability under real-world farming conditions. Additionally, future work should include the quantification of biologically active compounds in VC and VCT to understand better their role in plant physiology, nutrient dynamics, and pest resistance. Such data would strengthen the mechanistic understanding of how these amendments function and guide more targeted application strategies.